Switched to `check_ajax_referrer` for AJAX nonces
Refactored `plupload_image` field

Streamlined the `plupload_image` field .  Uses data attributes instead
of hidden input fields, cleaned up Javascript code, using
`check_ajax_referer` to check nonces.
Added parent parameter for `posts` field

If the `parent` paremeter for the `posts` field gets set to `true`, the
field defaults to single and uses the post's `post_parent` field as the
meta.  Can be used to make posts the children of other posts, even of
different post types